Microassembly of heterogeneous materials

ABSTRACT

A method for microassembly of heterogeneous materials comprises contacting a stamp with an ink disposed on a donor substrate to form an inked stamp, where the ink is reversibly bound to the stamp. The inked stamp is stamped onto a receiving substrate or onto an object on the receiving substrate, and the stamp is removed, thereby transferring the ink to the receiving substrate. The ink and the receiving substrate or the ink and the object are thermally joined, thereby forming a microassembly of heterogeneous materials. The ink may comprise a first material and the receiving substrate or the object may comprise a second material different from the first material.

RELATED APPLICATIONS

The present patent document claims the benefit of priority under 35U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/528,246,filed Jul. 3, 2017, which is hereby incorporated by reference in itsentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numberCMMI-1351370 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

TECHNICAL FIELD

The present disclosure is related generally to microfabrication and moreparticularly to a method of microassembly involving transfer printingand thermal joining.

BACKGROUND

Photolithography has been the dominant manufacturing technique inmicro/nano scale manufacturing due to its parallel and batch processnature. The convenience in scalability has significantly reduced theunit cost of individual microelectromechanical systems (MEMS) andintegrated chips (IC), enabling broader adaptation of devicesubiquitously found in our everyday lives. While enhanced exposuretechniques and the development of various photoresists (PR) have enabledthe miniaturization of IC devices down to the nanometer regime, the formfactor of such devices is generally limited to two dimensions owing tothe planar characteristics of the photolithography process.

Microscale devices with a three-dimensional (3D) form factor have foundapplication in microsystems such as radiofrequency (RF) MEMS switches,mechanical resonators, gyroscopes and pressure sensors, to name a few,due to the inherent advantages of 3D architectures. However, asindicated, fabricating devices with 3D form factors present a challengefor photolithographic patterning methods. For improved performance(e.g., higher sensitivity), such 3D devices may benefit from delicatelysuspended structures, which are typically achieved by removal of asacrificial layer buried beneath the structures. Another challenge isintegrating heterogeneous materials in MEMS and IC devices usingmicrofabrication given the disparate process parameters of differentclasses of materials.

Microassembly methods, such as transfer printing, may overcome drawbacksof monolithic microfabrication and permit novel 3D architectures to befabricated for microsystems. In the early stages of the development oftransfer printing, flat surfaced polydimethylsiloxane (PDMS) stamps wereutilized to transfer objects onto target receiving sites. Suchflat-surfaced PDMS stamps control adhesion by exploiting theviscoelastic nature of the stamp material, yet with boundedreversibility. For this reason, to promote successful transfer printing,highly adhesive receiver substrates were adopted. However, such highlyadhesive surfaces are generally polymeric, which limits applications forfunctional microsystems. The development of various stamps that exhibita high adhesion on/off ratio has been important to expand theapplicability of transfer printing for microsystem fabrication; however,further advancements are needed.

BRIEF SUMMARY

A method for microassembly of heterogeneous materials comprisescontacting a stamp with a solid-phase ink disposed on a donor substrateto form an inked stamp, where the solid-phase ink is reversibly bound tothe stamp. The inked stamp is stamped onto a receiving substrate or ontoan object on the receiving substrate, and the stamp is removed, therebytransferring the solid-phase ink to the receiving substrate. Thesolid-phase ink is then thermally joined with the object or thereceiving substrate to form a microassembly of heterogeneous materials.The object on the substrate may be a previously deposited solid-phaseink. Throughout the disclosure and claims, the terms “solid-phase ink”and “ink” may be used interchangeably.

A method of fabricating a polymeric ink on a donor substrate fortransfer printing includes: coating a sacrificial layer onto a siliconsubstrate; coating a polymeric material on the sacrificial layer andcuring to form a polymer layer; patterning the polymer layer to form apolymeric ink pattern; and removing the sacrificial layer, therebyforming the polymeric ink.

A method of fabricating a SiO₂ ink on a donor substrate for transferprinting includes: growing a SiO₂ layer on a silicon layer; patterningthe SiO₂ layer to form a SiO₂ ink pattern on the silicon layer;patterning the silicon layer to form a silicon pattern laterallysurrounding and underlying the SiO₂ ink pattern, thereby exposing aburied oxide layer away from the SiO₂ ink pattern; covering the SiO₂ inkpattern with photoresist and forming one or more photoresist anchors tothe buried oxide layer; and removing the silicon pattern, therebyforming the SiO₂ ink.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are schematics of an exemplary microassembly process,including preparation of inks (specifically, solid-phase inks) on donorsubstrates (FIG. 1A); alignment and contacting of a stamp with an ink toachieve reversible binding (FIG. 1B); picking up of the ink (FIG. 1C);transferring of the ink to a receiving substrate (printing) (FIG. 1D);and thermal joining to form the microassembly (FIG. 1E).

FIG. 2A is a schematic of an exemplary microassembly formed byindividually transferring different inks onto target sites on areceiving substrate followed by thermal processing.

FIG. 2B shows a scanning electron microscope (SEM) image of amicroassembly including double layer Si rings and SiO₂ discs.

FIG. 2C shows a SEM image of a microassembly including multiple layer Sidiscs and SU-8 blocks.

FIG. 2D shows a SEM image of the microassembly illustrated in FIG. 2A,which includes Si, SiO₂, Au, and SU-8 inks.

FIG. 2E shows a vertically aligned Si ring joined on a SU-8 block.

FIGS. 3A and 3B show schematics of a Si—Si blister test specimenassembled via microassembly, where FIG. 3A shows a Si disc ink,separately prepared on a donor substrate, transferred onto a receivingsubstrate and joined for hermetic sealing through Si—Si fusion bonding,and FIG. 3B shows a cross-sectional view of the assembled specimen ofthis example with dimensions.

FIG. 3C shows comprehensive data of joining strength with respect tomaterial pairs and thermal processing temperatures obtained throughblister tests.

FIG. 3D shows representative optical microscope images and finiteelement analysis (FEA) results for a Si—Si blister test specimen atthree pressure states that describe the central deflection in the microassembled Si disc.

FIGS. 4A-4F show schematics of silicon (Si) ink fabrication, including:a SOI wafer with desired device layer properties and 1 μm thick buriedoxide layer, shown in FIG. 4A; a patterned device Si layer, shown inFIG. 4B; photoresist (PR) patterning for selective undercut etching,shown in FIG. 4C; removal of exposed buried oxide layer and undercut ofoxide layer beneath the patterned Si layer, shown in FIG. 4D; formationof anchors within the undercut region, shown in FIG. 4E; and completeremoval of the buried oxide layer, resulting in the suspended Si layerbeing tethered by PR anchors, shown in FIG. 4F.

FIGS. 5A-5F show schematics of gold (Au) ink fabrication, including:growth of a ˜1 μm thermal oxide on Si wafer followed by titanium (Ti)and Au deposition, shown in FIG. 5A; etch back patterning of Au, shownin FIG. 5B; PR patterning for selective undercut etching, shown in FIG.5C; removal of exposed Ti and the buried oxide layer and slight undercutof the two layers beneath the patterned Au, shown in FIG. 5D; formationof anchors within the undercut region, shown in FIG. 5E; and completeremoval of Ti and the buried oxide layer resulting in the suspended Aulayer tethered by PR anchors, shown in FIG. 5F.

FIGS. 6A-6D show schematics of SU-8 ink fabrication, including: spincoating of PMMA on a Si wafer, shown in FIG. 6A; spin coating of SU-8,shown in FIG. 6B; photolithographic patterning, shown in FIG. 6C; andremoval of PMMA layer which reveals SU-8 inks adhered onto the substrateby mere surface adhesion force, shown in FIG. 6D.

FIGS. 7A-7F show schematics of silicon dioxide (SiO₂) ink fabrication,including: providing a SOI wafer, shown in FIG. 7A; thermal oxidation ofthe device layer, shown in FIG. 7B; patterning of the thermal oxideusing hydrofluoric acid (HF), shown in FIG. 7C; removal of the Si layerwhile ensuring no undercut region beneath the grown SiO₂, shown in FIG.7D; PR patterning to cover the patterned SiO₂ inks while anchoring tothe buried oxide layer, shown in FIG. 7E; and complete removal of deviceSi layer using xenon difluoride (XeF₂) isotopic etching, shown in FIG.7F.

FIGS. 8A-8E show schematics of fabrication of a microtippolydimethylsiloxane (PDMS) stamp, including: depositing a siliconnitride layer on a Si wafer and patterning into small squares that serveas bases of the microtip, shown in FIG. 8A; KOH etching of the Si waferand complete removal of the silicon nitride layer, shown in FIG. 8B;formation of SU8 walls that form a cavity, shown in FIG. 8C; filling thecavity with PDMS and curing, shown in FIG. 8D; and peeling off of thefully cured microtip stamp from the negative mold, shown in FIG. 8E.

FIGS. 9A-9F show a procedure of transfer printing using an elastomericmicrotip stamp, including: fabricating inks on a donor substrate (asdescribed above), shown in FIG. 9A; bringing the elastomeric microtipstamp in contact with an ink with a sufficiently high preload to formfully collapsed microtips, shown in FIG. 9B; rapidly retrieving thestamp such that the ink is adhered to the microtip stamp, and thepreviously fully collapsed microtips may be fully restored, resulting inminimal contact area at the tips, shown in FIG. 9C; delivering the inkonto a target receiving substrate, shown in FIG. 9D; bringing the ink incontact with the receiving surface, shown in FIG. 9E; and slowly raisingthe stamp that leaves the ink printed on a receiving substrate, shown inFIG. 9F.

FIGS. 10A-10C show schematics of processing various blister testreceiving substrates, including: fabrication flow of an Si receivingsubstrate for blister test, shown in FIG. 10A, deposition of Cr and Auor growth of SiO₂ on the Si receiving substrate to realize an Au or SiO₂coated surface, shown in FIG. 10B; patterning of a SU-8 rim on an Sisubstrate with an air inlet for SU-8 specimen, shown in FIG. 10C.

FIGS. 11A-11D show schematics of preparing blister test specimens viamicroassembly, including picking up a Si disc from a donor substrateprepared as depicted above, as shown in FIG. 11A; transferring the Sidisc ink onto a blister test receiving substrate, as shown in FIG. 11B;placing the Si disc ink, as shown in FIG. 11C; and thermally processingthe Si disc ink under appropriate conditions to form a robustly joinedinterface, as shown in FIG. 11D.

FIG. 12 shows a cross-sectional schematic of an exemplary blister testsetup, which may include a polymer jig, hose, pressure gauge, pump(syringe), a custom stage and a microscope.

FIGS. 13A-13D show the experimental set-up and results of measurementsto determine total resistance of a microassembly comprising Au inks,where a schematic of reference and assembled Au lines is provided inFIG. 13A; optical images of a reference Au line fabricated throughphotolithography and a connected Au line with an assembled Au ink areshown in FIGS. 13B and 13C; and I-V curves of the reference andassembled Au lines are presented in FIG. 13D.

DETAILED DESCRIPTION

Micro-assembly, which involves sequential transferring and joining ofindividual micro-scale materials, may overcome shortcomings ofmonolithic microfabrication and enable novel 3D architectures formicrosystems. Transfer printing, inspired from micro contact printing,may utilize the highly reversible surface adhesion of a polymericmanipulator (called a “stamp”) to deterministically transfer microscalesolid objects (called “solid-phase inks” or “inks”) in a dry manner. Theability to transfer inks from a donor substrate where inks are grown andprocessed to a receiving substrate where inks are thermally bondedreduces the complexity of manufacturing processes required forheterogeneous material integration. Furthermore, the dry nature oftransfer printing enhances the process compatibility with othermanufacturing techniques.

Microassembly of four different classes of materials semiconductors(e.g., Si), metals (e.g., Au), dielectrics (e.g., SiO₂) and polymers(e.g., SU-8) is described in this disclosure. For broader utilization ofthe microassembly process, SiO₂ and SU-8 materials are newly developedas inks. The microassembly method employs reversible adhesion-basedtransfer printing and thermal processing-based material joining. Theinterfacial joining characteristics between dissimilar materials arequantitatively studied through blister tests to validate the structuralintegrity of assembled structures and devices (microassemblies). Thiswork demonstrates the use of microassembly to fabricate 3D heterogeneousmicrosystems, with relevance not only to MEMS but also to electronics,photonics, metamaterials, and other fields. Furthermore, themicroassembly process can complement conventional micromachiningtechniques by transfer printing and joining an individual constituent ofa device at spatially organized sites for enhanced performance or novelfunctionalities.

FIGS. 1A-1E schematically illustrates an exemplary microassembly processthat can be divided into three sequential steps: preparing (FIG. 1A),transferring (FIGS. 1B-1D), and thermal joining of inks (FIG. 1E). Theinks or ink materials prepared in this disclosure include Si, SiO₂, Auand an epoxy polymer (e.g., SU-8). One or more inks (e.g., a discreteink or an array of inks) may be prepared for easy retrieval from a donorsubstrate during transfer printing, as described below in reference toFIGS. 4A-7F. An ink prepared on a donor substrate may be transferredonto a target area of a receiving substrate utilizing a stamp, such as amicrotip stamp as illustrated in FIGS. 1B-1D and elsewhere in thisdisclosure, and as described in U.S. Pat. No. 9,412,727, which is herebyincorporated by reference in its entirety. The receiving substrate withthe transferred ink is subsequently thermally processed to join the inkand the target area of the substrate by fusion, eutectic or adhesivebonding under appropriate conditions (e.g., see Table 1). Thistransferring and joining cycle may be repeated until a microassemblyhaving a predetermined 3D structure is achieved, as shown for example inFIGS. 2A-2E. These exemplary 3D assembled structures realized from 2Dinks at the microscale reveal unparalleled heterogeneous materialassembly capabilities that can further be exploited in variousapplications.

Referring now to FIGS. 1A-1E, the method for microassembly ofheterogeneous materials may include preparing an ink (or multiple inks)102 on a donor substrate 104, as indicated in FIG. 1A, and contacting astamp 106 with the ink 102, as shown in FIG. 1B, thereby forming aninked stamp 108, as shown in FIG. 1C. The ink 102 comprises a firstmaterial, such as Si, Au, SiO₂ or SU-8 or another polymer. The stamp 106is capable of reversible binding of the ink 102.

The inked stamp 108 (i.e., the stamp 106 and the ink 102 reversiblybound to the stamp 106) is removed from the donor substrate 104, asillustrated in FIG. 1C, and aligned with a target area on a receivingsubstrate 110. The ink 102 may be printed directly onto the receivingsubstrate 110 or onto an object (e.g., a previously deposited ink 102)on the receiving substrate 110. The object and/or the receivingsubstrate 110 may comprise a second material different from the firstmaterial. The second material may be selected from silicon, gold, SiO₂and/or SU-8 or another polymer. The ink 102 is stamped onto the objector the receiving substrate 110, as shown in FIG. 1D, and the stamp 106is removed, thereby transferring the ink 102 to the receiving substrate110. After transfer printing, the ink 102 and the object and/orreceiving substrate 110 are heated to effect bonding (i.e., thermallyjoined), thereby forming a microassembly 100 of heterogeneous materials,as shown in FIG. 1E.

As indicated above, the stamp 106 is capable of reversibly binding theink 102. More specifically, the stamp 106 may comprise a material thatexhibits minimal adhesion to the ink 102; the adhesion is sufficient toallow for transportation of the ink 102 on the stamp 106 but alsorelease of the ink 102 upon removal of the stamp 106 from the receivingsubstrate 110. For example, the stamp 106 may comprise a material withviscoelastic properties, such as polydimethylsiloxane (PDMS) or anotherpolymer.

The method may further include repeating the contacting and stamping toadd additional inks 102 to the receiving substrate 110. Themicroassembly 100 may include at least two of the inks 102 stacked onthe receiving substrate 110, as shown schematically in FIG. 1E and inFIG. 2A. Depending on the arrangement of the inks 102, the microassembly100, 200 may include an overhanging or suspended portion or another 3Dgeometry that is difficult or impossible to fabricate using conventionalphotolithographic patterning.

FIGS. 2B-2E are scanning electron microscopy (SEM) images ofrepresentative microassemblies formed by transfer printing and thermaljoining; FIG. 2B shows a microassembly comprising double layer Si ringsand SiO₂ discs; FIG. 2C shows a microassembly including multiple layerSi discs and SU-8 blocks; FIG. 2D shows a microassembly composed of Si,SiO₂, Au, and SU-8 inks of various shapes; and FIG. 2E shows amicroassembly including a vertically aligned Si ring joined on a SU-8block.

The repeating of the contacting and stamping to incorporate multipleinks 102 in the microassembly 100,200 (e.g., as shown in FIG. 2A) may becarried out prior to thermal joining; in other words, the thermaljoining may take place in a single step after all of the inks 102 areplaced in a predetermined arrangement on the receiving substrate 110.Alternatively, after thermally bonding the (first) ink 102 to thereceiving substrate 110, the thermal joining may be repeated at suitabletemperatures as additional inks 102 are stamped onto the receivingsubstrate 110, either directly onto the receiving substrate 110 or ontoobjects (such as previously deposited inks 102) on the receivingsubstrate 110. In other words, the thermal joining may be carried out inseparate steps at different temperatures as additional inks 102 areprinted. As would be recognized by the skilled artisan, the thermaljoining may include ink-to-receiving substrate bonding and/or ink-to-inkbonding with the printing of additional inks 102. It is also worthwhileto note that the temperature of individual joining steps may beconsidered when the sequence of assembly and thermal joining isdetermined, since inks comprising materials such as SU-8 or Au may notbe able to withstand the higher temperatures required for joining Si—Sior Si—SiO₂, for example.

Thus, the ink(s) 102 and the receiving substrate 110 may be thermallyjoined by heating at a suitable temperature determined in full or inpart by the materials being joined. In one example, the first (ink)material comprises SiO₂ and the second (receiving substrate) materialmay be SU-8 or Si. In another example, the first material comprises SU-8and the second material may be SiO₂, Si, and Au. Examples of thermaljoining conditions for different ink-receiving substrate pairs are setforth in Table 1. The bonding mechanism depends on the materials and maybe described as fusion, eutectic or adhesive bonding, as summarized inTable 2.

Generally speaking, thermal joining is carried out at a temperature in arange of about 125° C. to about 1000° C., about 150° C. to about 1000°C., about 300° C. to about 1000° C., or about 600° C. to about 1000° C.The thermal joining may take place at a temperature of at least about125° C., at least about 150° C., at least about 300° C., at least about500° C., at least about 600° C., or at least about 900° C., depending onthe materials involved. The thermal joining temperature may also be nogreater than 200° C., no greater than 400° C., no greater than 700° C.,or no greater than 1100° C., depending on the materials involved. Forexample, a SiO₂ ink may be joined to a Si receiving substrate by heatingat a temperature in a range from about 900° C. to 1100° C. If, however,the Si receiving substrate first undergoes activation with oxygen,thermal joining with the SiO₂ ink may be carried out at a lowertemperature in a range from about 500° C. to about 700° C. In anotherexample, the SiO₂ ink may be joined to a SU-8 receiving substrate byheating at a temperature in range from about 125° C. to about 175° C. Inyet another example, a SU-8 ink may be joined to a receiving substratecomprising Si, SiO₂ or Au by heating at a temperature in a range fromabout 125° C. to about 175° C. As would be recognized by the skilledartisan, the temperature ranges given above apply to the material pairsregardless of which is the ink and which is the receiving substrate.

TABLE 1 Exemplary joining conditions for construction of 3Dmicroassemblies. Receiving substrate Ink material material JoiningCondition (Examples) Si Si Si ink is directly transfer printed onto atarget Si surface and thermally processed in a furnace at 1000° C. for10 min with 5 sec ramping. Si SiO₂ For high temperature (1000° C.)joining, a SiO₂ ink is directly printed onto a Si surface and thermallyprocessed in a furnace at 1000° C. for 10 min with 5 sec ramping. Forlower temperature (600° C.) joining, a SiO₂ ink is first transferprinted onto a Si substrate, which undergoes O₂ descumming process (O₂20 sccm, 150 mTorr, 200 W, 5 min) to remove photoresist (PR) that coversthe SiO₂ ink. The ink is then transfer printed onto an activated (O₂, 20sccm, 150 mTorr, 100 W, 30 sec) Si surface. The substrate is then placedand thermally processed in a furnace at 600° C. for 10 min with 5 secramping. Si Au The surface of Si is cleaned with HF for removal ofnative oxide layer followed with transfer printing of an Au ink. The Auink is transfer printed within short period of time after the HFtreatment. The transfer printed sample is then placed in a furnace andthermally processed at 365° C. for 10 min with 5 sec ramping. Si SU-8SU-8 ink is directly printed on a Si surface and thermally processed ina furnace at 150° C. for 10 min with 10 min ramping. SiO₂ Si Si ink isdirectly printed onto a SiO₂ surface and thermally processed in afurnace at 1000° C. for 10 min with 5 sec ramping. Au Au Au ink isprinted onto a clean Au surface with a moderate pressure for moreintimate contact. SU-8 Si, Au, A desired ink is printed and thermallySiO₂ processed in a furnace at 150° C. for 10 min with 10 min ramping.Si, Au, SU-8 SU-8 ink is printed on a receiving substrate SiO₂ andthermally processed in a furnace at 150° C. for 10 min with 10 minramping.

TABLE 2 Diverse joining mechanisms utilized for joining differentmaterial combinations Joining Materials Joining Techniques Si Si, SiO₂Fusion Si Au Eutectic Au Au Cold welding SU-8 Si, Au, SiO₂ Adhesive

Generally speaking, the heating may be carried out for 30 minutes orless, 20 minutes or less, 10 minutes or less, or five minutes or less,and typically for at least about 0.5 minute or at least about 1 minuteto effect bonding. Typically, the heating/thermal joining is carried outfor 5 to 15 minutes. The thermal joining may take place in air or acontrolled environment, e.g., inert gas or vacuum.

An advantage of the method is that application of pressure is notrequired during thermal joining. In contrast, in wafer scale joining,two objects are brought into contact and exposed to a preload duringheating to promote intimate contact. The process does not requireexternal forces during thermal processing since typical inks aresignificantly smaller than wafers, such that intermolecular forcesbetween inks are strong enough to maintain sufficient surface contact.These dominant intermolecular forces may originate from reduced defectspresent in the small contact area when compared to wafer-scale bondingareas.

To demonstrate the integrity of bonded interfaces, microassembliesformed by transfer printing and thermal joining are evaluated withblister testing as described below, Experimental assessment of theinterfacial joining strength between printed inks/substrates can notonly ensure the robustness of assemblies constructed throughmicroassembly but also allow a comparison between various mechanisms tojoin different materials at the microscale. Blister tests have beensuccessfully utilized to characterize the adhesion of thin films formedon Si substrates and are utilized in this disclosure for measuring thejoining strength at Si—Si, Si—SiO₂, Si—Au and Si-SU-8 interfaces of themicroassemblies.

FIGS. 3A and 3B show a procedure to make an exemplary Si ink for ablister test and its dimensions, respectively. The Si ink, which has theshape of a disc, is prepared on a donor substrate, transferred onto areceiving substrate (which may be Si or coated with SiO₂, Au or SU-8),and joined for hermetic sealing. The pressure inside a hermeticallysealed microcavity 320, shown in FIG. 3B increases in a controlledmanner using a syringe pump, which induces the delamination of an ink, asilicon ink in this example, from the rim structure on a receivingsubstrate at critical pressure that satisfies Griffith's fracturecriterion:G _(c) =G(p _(c))=0.625p _(c) d _(c)  (1)

where G_(c) is a function of material properties of the ink and thereceiving substrate and is termed critical energy release rate ortoughness, which indicates the material's resistance to fracture alongany given path. Provided that the ink delaminates along the joininginterface, the corresponding G_(c) indicates the toughness of thejoining. The energy release rate G, on the other hand, is a loadingparameter indicating the driving force for fracture. For the specimengeometry here, G is simply a function of the applied pressure p_(c) andthe central deflection of the ink d_(c). Finite element analysis (FEA)is conducted to determine d_(c) that is a function of measured p_(c),specimen dimensions and material properties. At the moment when the Siink delamination occurs, G reaches the toughness G_(c). Upon failure,the critical pressure P_(c) is measured and the corresponding deflectiond_(c) is obtained through simulation, as shown in FIG. 3D. By combiningthe experimentally obtained Pc and simulated d_(c) values, the criticalenergy release rate G_(e) can be formulated.

A full description of blister test specimen fabrication, joiningconditions and testing procedures is provided below in reference toFIGS. 10A-12.

Three specimens are tested per each joining material pair and theresultant G_(c) are plotted in FIG. 3C with respect to their thermalprocessing temperatures. Two different joining conditions areinvestigated in both Si—SiO₂ and Si-SU-8 pairs to compare the optimaland as conducted cases (Table 3). FIG. 3C shows data of joining strengthversus thermal processing temperature for the bonded material pairs,including Si to Si, Si to SiO₂, Si to surface treated SiO₂, Si to Au, Sito SU-8, and Si to acetone treated SU-8, as determined from the blistertests. The data show that the joining strength ranges from about 0.3J/m² to 4.5 J/m², which is compatible with wafer scale Si—Sifusion-bonded joining strength. Remarkably, all joining strength data ofFIG. 3C are similar to or higher than the toughness data for siliconwafer bonding measured elsewhere, although the joining here is achievedwithout external forces during thermal processing, in contrast to waferbonding techniques. (There is one exception: the case of the Si—Aublister test specimen, where ˜150 kPa of external pressure is appliedduring thermal processing to form hermetic sealing in the microcavity.Therefore, the measured joining strength at the Si—Au interface shown inFIG. 3C may be understood to be the upper bound of the actual joiningstrength.) FIG. 3D provides optical images of an assembled Si ink uponpressuring and FEA results where the ink is ruptured prior todelamination from the underneath Si rim, indicating that the measuredvalue for a Si—Si pair in FIG. 3C is the lower bound of the actualjoining strength.

While thermal processing conditions for material joining inmicroassembly may be inspired by conventional wafer bonding techniques,optimal wafer-scale thermal processing conditions do not necessarilyapply to microassembly processes. For example, an assembled Si—SiO₂structure may fail to retain its original structure after thermalprocessing because of different thermal expansion coefficients betweenSi and SiO₂. In this case, a relatively low thermal processingtemperature (600° C.) in conjunction with oxygen plasma surfaceactivation may be exercised in microassembly. To reproduce such aprocess for a blister test specimen, a SiO₂ coated receiving substratemay be activated using oxygen plasma followed by transfer printing andthermal processing. The test results yield 0.3 J/m², which is lower thanfor Si—SiO₂ pair joined at the temperature of 1000° C., but still on parwith other known wafer-scale joining strength values. Similarly, thejoining strength between Si and acetone-treated SU-8 is separatelystudied since SU-8 inks for microassembly are prepared by releasing SU-8inks in acetone bath. As expected, the acetone treatment on SU-8 canreduce the joining strength in comparison with an unadulterated Si toSU-8 interface, but it is still on the same order of magnitude with allother obtained joining strength data. In summary, the joining strengthdata obtained through blister tests (as shown in FIG. 3C) stronglysupport the device level capabilities of the microassembly technique.

TABLE 3 Thermal processing conditions for joining Si inks to differentreceiving substrates. Receiving Surface Materials Joining Conditions SiA Si ink is transfer printed and thermally processed in a furnace at1000° C. for 10 min with 5 sec ramping. SiO₂ Condition 1: A Si ink istransfer printed and thermally processed in a furnace at 1000° C. for 10min with 5 sec ramping. Condition 2: A receiving SiO₂ surface isactivated (O₂ 20 sccm, 150 mTorr, 100 W, 30 sec). Afterwards, a Si inkis transfer printed and thermally processed at 600° C. for 10 min with 5sec ramping. Au Any native oxide on a Si ink is removed in a HF bath.The Si ink is transfer printed and thermally processed at 365° C. for 10min with 5 sec ramping. SU8 Condition 1: A Si ink is directly transferprinted and thermally processed in a furnace at 150° C. for 10 min with10 min ramping. Condition 2: A SU8 receiving substrate is baked at 110°C. for 1 min followed with immersing in acetone bath for 1 min beforethermal processing at 150° C. for 10 min with 10

The applications of microassembly are enhanced by the development ofmethods to fabricate SU-8 and SiO₂ as inks, as shown in FIGS. 6A-6D andFIGS. 7A-7F, respectively, FIGS. 4A-5F, which are discussed in theExamples below, illustrate fabrication of Si and Au inks. Each materialcan require a unique processing approach to successfully form an inksuitable for transfer printing.

FIGS. 7A-7F illustrate an exemplary process flow of the SiO₂ ink. Theprocess flow may begin with selection of a silicon on insulator (SOI)wafer 730, as shown in FIG. 7A, which undergoes thermal processing togrow a thermal oxide on the device layer 732. In order to establish asufficient thickness of the thermal oxide as well as the underlying Silayer 732, which becomes a sacrificial layer, the device layer thicknessof the SOI wafer may be carefully selected. As illustrated in FIG. 7B,the 501 wafer 730 is thermally oxidized to grow a desired thickness ofSiO₂ 734 over the device layer 732. Growth of the thermal oxide 734 isfollowed by patterning of the oxide layer to form a SiO₂ ink pattern 736on the silicon layer 732 (e.g., using reactive ion etching (RIE)), asindicated in FIG. 7C. After patterning the SiO₂ layer 734, the device(silicon) layer 732 of the SOI wafer 730 is removed, as illustrated inFIG. 7D, except for a portion laterally surrounding (and underlying) thepreviously patterned SiO₂. In other words, the silicon layer 732 ispatterned to form a silicon pattern 738 laterally surrounding andunderlying the SiO₂ ink pattern 736 and to expose a buried oxide layer740 away from the SiO₂ ink pattern 736. Accordingly, the patterning ofthe silicon layer 732 is carried out with a pattern larger in lateraldimension than that employed to form the SiO₂ ink pattern 736, and mayentail reactive ion etching. A layer of photoresist is then applied andpatterned to form photoresist anchor(s) 744 tethering the photoresist742 on the SiO₂ ink pattern 736 to the substrate 730, as shown in FIG.7E. As can be seen, the SiO₂ ink pattern 736 is completely covered with(or conformally coated with) the photoresist 742 and the photoresistanchor(s) 744 extend to the buried oxide layer 740. Once the photoresistanchors 744 are formed, the silicon pattern 738 laterally surroundingand underlying the SiO₂ ink pattern 736 may be removed through xenondifluoride (XeF₂) isotopic etching, which results in suspended SiO₂ink(s), which are present but not visible in FIG. 7F due to theoverlying photoresist 742. The photoresist 742 and photoresist anchors744 need not be removed prior to transfer printing and thermal joiningof the SiO₂, although removal to uncover the SiO₂ ink(s) 746 can becarried out prior to transfer printing if desired.

SU-8 is processed to ink format as illustrated in FIGS. 6A-6D. Theincludes coating a sacrificial layer 650 onto a silicon substrate 652,as shown in FIG. 6A, and coating a polymeric material (e.g., anepoxy-based polymer such as SU-8) on the sacrificial layer 650 andcuring to form a polymer layer 654, as shown in FIG. 6B. For example, apolymethyl methacrylate (PMMA) sacrificial layer 650 may be spin coatedonto a silicon substrate 652, followed by spin coating and curing of aSU-8 polymer layer 654. The thickness of the SU-8 coated onto thesubstrate is determined by the desired thickness after microassembly.The polymer layer 654 is then patterned to form a polymeric ink pattern656, as shown in FIG. 6C, where the patterning may include exposing thepolymer layer to light through a pattern mask, baking after theexposing, and developing after the baking. Finally, the sacrificiallayer 650 is removed by, for example, exposure to a solvent, therebyforming a polymeric (e.g., SU-8) ink 658 on the silicon substrate 652for transfer printing. The removal may entail submersion of thesacrificial layer in an acetone bath. During this sacrificial removalstep, it may be critical for the acetone bath to remain undisruptedsince the polymeric ink is not anchored as is the SiO₂ ink describedabove; thus, the polymeric ink remains on the substrate due to meresurface interaction, which may be infinitesimally small to facilitatetransfer printing using a stamp manipulator.

EXAMPLES

Microassembly Procedure

Si, Au, SiO₂ and SU-8 inks may be prepared on individual donorsubstrates as shown in FIGS. 4A-7F. FIGS. 8A-8E show an exemplaryfabrication process of a stamp, as described below.

An exemplary donor substrate 104 including four inks 102 is shown inFIG. 9A. A microtip stamp 106 made of PDMS in this example is brought tocontact with one of the inks 102 with high preload such that allmicrotips are fully collapsed, as shown in FIG. 9B. Rapid retrieval ofthe microtip stamp 106 allows the ink 102 to be separated from the donorsubstrate 104 and adhere to the microtip stamp 106 (forming inked stamp108), as shown in FIG. 9C. Once the preload is removed, the stamp 106returns to its original microtip configuration, which results in minimaladhesion between the retrieved ink 102 and the stamp 106 due to thereduced contact area. Subsequently, the stamp 106 with the ink 102 isdelivered to a target area on a receiving substrate 110 (FIG. 9D) andbrought to contact with the substrate 110 with minimal preload, as shownin FIG. 9E. The stamp 106 is then separated from the substrate 110 atlow speed, which leaves the ink 102 on the target area of the receivingsubstrate 110 due to the stronger intermolecular interaction between thereceiving substrate 110 and the ink 102, as shown in FIG. 9F. Followingtransfer printing of the ink 102, the receiving substrate 110 and theink 102 are thermally processed (e.g., as described in Table 1) to jointhe ink 102 and the substrate 110.

Microassembly of Blister Test Specimens

Receiving substrates with rims covered or formed by four differentmaterials are made as depicted in FIGS. 10A-10C. On a separate donorsubstrate, Si disc inks are fabricated and these inks are assembled onreceiving substrates as illustrated in FIGS. 11A-11D. FIG. 12 describesthe test setup where the hermetically sealed specimens are pressurizeduntil the inks (e.g., Si disc inks) are delaminated or ruptured. Thejoining conditions are provided in Table 3.

Assembly Procedure of a Vertical Si Ring on a SU-8 Block

Si ring-shaped inks are prepared on a donor substrate and retrieved by aPDMS microtip stamp. On one side of the retrieved ring shaped ink,normal force is applied in a horizontal direction, which causesdelamination of ink from the stamp and the ink is attached vertically onan adjacent side of the stamp. Afterwards, the vertically adhering inkis transferred and joined on an SU-8 block to form a 3D Si/SU-8structure, as shown in FIG. 2E.

Si Ink Fabrication

1. Determine an SOI wafer based on top Si layer thickness requirementand buried oxide (BOX) layer of 1 μm (Ultrasil), as shown in FIG. 4A.

2. Pattern AZ 5214 photoresist (PR: AZ electronic materials) by firstspinning at 3000 rpm for 30 seconds, followed with soft baking at 110°C. for 1 minute. Once the PR is soft baked, expose using I-line UV (KarlSuss MJB 3) for the dose of 130 mJ/cm² and develop using 917 MIFdeveloper (AZ electronic materials) for approximately 20 seconds.

3. Pattern Si device layer using reactive ion etching (RIE: PlasmaTherm:40 sccm SF6, 50 mTorr, 100 W, 3 minutes), as shown in FIG. 4B.

4. Remove photoresist masking layer and pattern second mask patternusing AZ 5214 PR for selective undercut protection. Identical procedureas step 2 is used for patterning the PR.

5. Post exposure bake at 110° C. for 60 seconds, as shown in FIG. 4C.

6. Place the substrate in 49% Hydrofluoric Acid (HF: Sigma-Aldrich) for55 seconds, as shown in FIG. 4D.

7. Remove photoresist and construct AZ 5214 PR anchor through identicalprocedure as in step 2, as shown in FIG. 4E. Since the PR anchors remainonly the undercut-etched region, mask pattern is unnecessary. Floodexposure (ABM Flood Exposure Model 60) is used in this case with samedosage as in step 2.

8. Post exposure bake of the PR anchors at 110° C. for 90 seconds.

9. Leave the substrate for sufficient time in 49% HF bath for completeremoval of BOX layer underneath the Si pattern, as indicated in FIG. 4F.

Au Ink Fabrication

1. An Si wafer (University wafer) is placed inside furnace to grow ˜1 μmof thermal oxide layer (Lindberg Hevi-Duty Lancer M-300) at 1100° C.with 6 sccm O₂ for 48 hours.

2. 5 nm of Ti and 400 nm of Au are sputter (AJA ATC ORION 8HV) deposited(20 sccm Ar, 5×10⁻³ Torr, 300 W, 1 min for Ti and 20 min for Au), asindicated in FIG. 5A.

3. Pattern AZ 5214 photoresist (PR: AZ electronic materials) by firstspinning at 3000 rpm for 30 seconds, followed with soft baking at 110°C. for 1 minute. Once the PR is soft baked, expose using I-line UV (KarlSuss MJB 3) for the dose of 130 mJ/cm² and develop using 917 MIFdeveloper (AZ electronic materials) for approximately 20 seconds.

4. Post exposure bake at 110° C. for 1 minute.

5. Place the PR patterned substrate in Au etchant (Sigma-Aldrich) forapproximately 2 minutes to pattern Au followed with PR removal, asindicated in FIG. 5B.

6. Pattern second mask pattern using AZ 5214 PR for selective undercutprotection. Identical procedure as step 2 is used.

7. Post exposure bake at 110° C. for 60 seconds, as shown in FIG. 5C.

8. Place the substrate in 49% Hydrofluoric Acid (HF: Sigma-Aldrich) for55 seconds, as indicated in FIG. 5D.

9. Remove photoresist and construct AZ 5214 PR anchor through identicalprocedure as in 2, as indicated in FIG. 5E. Since the PR anchors remainonly the undercut-etched region, mask pattern is unnecessary. Therefore,flood exposure (ABM Flood Exposure Model 60) is used in this case withsame dosage as in step 2.

10. Post exposure bake of the PR anchors at 110° C. for 90 seconds.

11. Leave the substrate for sufficient time in 49% HF bath for completeremoval of buried oxide and Ti sacrificial layer underneath the Aupattern, as indicated in FIG. 5F.

SU-8 Ink Fabrication

1. Spin coat 495 poly(methylmethacrylate) A resist with 6% in Anisole(PMMA: Microchem) at 3000 rpm and soft bake at 180° C. for 1 minutes, asshown in FIG. 6A.

2. Spin coat SU8-50 (Microchem) at 3000 rpm and soft bake on hot platefor 65° C. for 6 minutes and 95° C. for 20 minutes, as indicated in FIG.6B.

3. Using H-line flood exposure (ABM Flood Exposure Model 60), exposeSU8-50 through a pattern mask for 200 mJ/cm².

4. Post exposure bake at 65° C. for 1 minutes and 95° C. for 5 minutes.

5. Develop using SU-8 developer for 6-10 minutes (MicroChem), as shownin FIG. 6C.

Submerge the substrate in acetone bath for 1 min for complete removal ofPMMA sacrificial layer, as indicated in FIG. 6D.

SiO₂ Ink Fabrication

1. An SOI wafer (Ultrasil) with device layer with 1.5 μm (FIG. 7A) isplaced inside furnace to grow ˜1 μm of thermal oxide layer (LindbergHevi-Duty Lancer M-300) at 1100° C. with 6 sccm O₂ for 48 hours, asshown in FIG. 7B.

2. Pattern AZ 5214 photoresist (PR: AZ electronic materials) by firstspinning at 3000 rpm for 30 seconds, followed with soft baking at 110°C. for 1 minute. Once the PR is soft baked, expose using I-line UV (SussMJB 3) for the dose of 130 mJ/cm² and develop using 917 MIF developer(AZ electronic materials) for approximately 20 seconds.

3. Post exposure bake at 110° C. for 1 minute.

4. Place the substrate in 49% HF (Sigma-Aldrich) for 55 seconds, asindicated in FIG. 7C.

5. Remove PR masking layer and Pattern AZ 5214 PR for device layerpatterning following the identical procedure as in step 2. This secondPR pattern is designed to be approximately 10% larger in lateraldimension than previous oxide pattern in order to protect SiO₂ layerduring RIE process.

6. Pattern Si device layer using reactive ion etching (RIE: PlasmaTherm:40 sccm SF₆, 50 mTorr, 100 W, 3 minutes) and remove PR, as indicated inFIG. 7D.

7. Construct AZ 5214 PR anchor using the identical procedure as in step2. The pattern needs to completely cover patterned thermally grown SiO₂layer as well as some anchors that extends out to buried oxide layer, asshown in FIG. 7E.

8. Place the substrate in XeF₂ etcher (Xactix) for 30 cycles that runs 3Torr XeF₂, 50 seconds per cycle for complete removal of Si device layerthat served as sacrificial layer, as indicated in FIG. 7F.

Elastomeric Microtip Stamp Fabrication

1. On {100} Si substrate (University Wafer), deposit 50 nm of Si₃N₄using PECVD (STS systems USA Inc) with medium frequency (1960 sccm N₂,40 sccm SiH₄, 35 sccm NH₃, 650 mTorr, 300° C. platen and 240° C.showerhead temperatures, 13.56 MHz 20 W 6 seconds and 380 KHz, 20 W 2seconds per cycle, total of 37 cycles).

2. Pattern AZ 5214 photoresist (PR: AZ electronic materials) by firstspinning at 3000 rpm for 30 seconds, followed with soft baking at 110°C. for 1 minute. Once the PR is soft baked, expose using I-line UV (SussMJB 3) for the dose of 130 mJ/cm² and develop using 917 MIF developer(AZ electronic materials) for approximately 20 seconds.

3. Post exposure bake at 110° C. for 1 min.

4. Etch Si₃N₄ using 10:1 buffered oxide etchant (BOE: Sigma-Aldrich) bysubmerging the substrate in the BOE bath for 150 seconds and remove AZ5214 PR, as indicated in FIG. 8A.

5. Place the substrate in potassium hydroxide (KOH: Fisher Scientific),isopropyl alcohol (IPA: Fisher Scientific) and deionized water (DI)mixture bath (70 g KOH, 70 ml IPA and 170 ml DI).

6. Place the KOH bath on 80° C. hot plate for ˜4 hours.

7. Remove the Si₃N₄ masking layer by submerging in HF (Sigma-Aldrich)bath for 150 seconds, as indicated in FIG. 8B.

8. Spin coat SU8-50 (Microchem) at 3000 rpm and soft bake on hot platefor 65° C. for 6 minutes and 95° C. for 20 minutes.

9. Using H-line flood exposure (ABM Flood Exposure Model 60), exposeSU8-50 through a pattern mask for 200 mJ/cm².

10. Post exposure bake at 65° C. for 1 minutes and 95° C. for 5 minutes.

11. Develop using SU-8 developer for 6-10 minutes (MicroChem).

12. Place the substrate vertically inside desiccator and apply 3-5droplets of (Tridencafluoro-1,1,2,2-Tetrahydrooctyl)-1-Trichlorosilane(Trichlorosilane: United Chemical Technology) inside the desiccator.

13. Leave the substrate for 1 hour in vacuum. A monolayer oftrichlorosilane is deposited on the surface of the Si/SU-8 substrate asshown in FIG. 8C.

14. Mix polydimethylsiloxane (PDMS: Sylguard 184, Dow Corning)pre-polymer base and cross-linking agent at 5:1 ratio.

15. Stir the mixture thoroughly and place the mixture inside a vacuumjar for 30 minutes. This step removes any micro scale bubbles that canbe trapped inside the mixture.

16. Slowly pour the PDMS mixture over Si/SU-8 substrate.

17. Cure the PDMS by placing the substrate inside 70° C. oven for 2hours, as indicated in FIG. 8D.

18. Slowly peel off the PDMS, as indicated in FIG. 8E, and remove theexcessive PDMS using razor blade if necessary.

Transfer Printing Using Elastomeric Microtip Stamp

1. Prepare inks with desired material and dimensions, as shown in FIG.9A, following procedures described above.

2. Align microtip elastomeric stamp fabricated as described above with atarget ink that will be transferred.

3. Apply a preload such that all microtips are fully collapsed resultingincreased surface contact area, as indicated in FIG. 9B.

4. Rapidly retract the elastomeric microtip stamp for ink pick up, asindicated in FIG. 9C. The removed preload causes the microtips torestore to original configuration that results reduced contact areabetween stamp and the retrieved ink.

5. Transfer the ink to desired receiving site as indicated in FIG. 9D.

6. Contact the ink with the receiving site using marginal preload, asshown in FIG. 9E.

7. Raise the stamp slowly, as shown in FIG. 9F, such that the ink isprinted on substrate.

Blister Test Receiving Substrate Fabrication

I. Si Receiving Substrate, as Illustrated in FIG. 10A:

1. Deposit 200 nm of SiO₂ using PECVD (PlasmaTherm) on Si wafer.

2. Pattern SiO₂ layer to ring shape by photolithography, RIE etching(Axic), 40 sccm CF₄, 200 W, 35 mTorr, 20 minutes), and photoresiststripping.

3. Pattern a 100 μm square via hole opening by photolithography, etchthrough the wafer by DRIE (STS Pegasus DRIE, C4F8 200 sccm, 100 mTorr,2000 W coil power for 4 sec passivation step, SF₆ 450 sccm and O₂ 45sccm, 100 mTorr, 2800 W coil power, 40 W platen power for 7 sec etchstep), and strip photoresist.

4. Etch silicon using DRIE with oxide mask on (STS Pegasus DRIE).

5. Dip the silicon substrate into hydrofluoric acid to remove SiO₂ mask.

6. Cleave the wafer into chips.

II. Au and SiO₂ Receiving Substrates, as Illustrated in FIG. 10B

1. Both Au and SiO₂ begin with a Si receiving substrate prepared asdescribed above.

2. For Au receiving sample, 400 nm Au is sputter (AJA ATC ORION 8HV)deposited (20 sccm Ar, 5×10⁻³ Torr, 300 W, 20 min for Au).

3. For SiO₂ receiving substrate, the Si receiving substrate is placedinside furnace (Lindberg Hevi-Duty Lancer M-300) at 1100° C. with 6 sccmO₂ for 24 hours to thermally grow 700 nm of SiO₂.

III. SU-8 Receiving Substrate, as Illustrated in FIG. 10C

1. Via hole is constructed using DRIE (STS Pegasus DRIE).

2. Spin coat SU8-50 (Microchem) at 3000 rpm and soft bake on hot platefor 65° C. for 6 minutes and 95° C. for 20 minutes.

3. Using i-line flood exposure UV lithography (Karl Suss MJB3 maskaligner), expose SU8-50 through a pattern mask for 200 mJ/cm².

4. Post exposure bake at 65° C. for 1 minutes and 95° C. for 5 minutes.

5. Develop using SU-8 developer for 6-10 minutes (MicroChem).

TABLE 4 Summary of joining rim dimensions and joining surfaces. JoiningRim Joining Rim Inner Diameter Outer Material (microns) Diameter JoiningSurface Si receiving surface 600 800 Si rim Au-coated receiving 600 800400 nm-thick Au surface SiO₂-grown receiving 600 800 700 nm-thick oxidesurface SU8-patterned 600 800 40 micron- receiving thick SU8-50 surface

Blister Test Specimen Assembly Via Micro-Lego

1. Prepare 3 μm thick, 900 μm diameter Si ink and microtip stampfollowing the procedures described above.

2. Bring the Si ink and the stamp together with high preload and rapidlyraise the stamp to retrieve the Si disc ink, as shown in FIG. 11A.

3. Transfer the Si ink onto a receiving substrate prepared as describedabove, as shown in FIG. 11B.

4. Print the ink with precision, as indicated in FIG. 11C.

5. Thermally process the specimen for hermetic joining throughout thecontact area, as shown in FIG. 11D.

Although the present invention has been described in considerable detailwith reference to certain embodiments thereof, other embodiments arepossible without departing from the present invention. The spirit andscope of the appended claims should not be limited, therefore, to thedescription of the preferred embodiments contained herein. Allembodiments that come within the meaning of the claims, either literallyor by equivalence, are intended to be embraced therein.

Furthermore, the advantages described above are not necessarily the onlyadvantages of the invention, and it is not necessarily expected that allof the described advantages will be achieved with every embodiment ofthe invention.

The invention claimed is:
 1. A method for microassembly of heterogeneousmaterials, the method comprising: contacting a stamp with a solid-phaseink disposed on a donor substrate to form an inked stamp, where thesolid-phase ink is reversibly bound to the stamp; stamping the inkedstamp onto a receiving substrate or onto an object on the receivingsubstrate, wherein the solid-phase ink comprises a first material andthe receiving substrate or the object comprises a second materialdifferent from the first material; removing the stamp, therebytransferring the solid-phase ink to the receiving substrate; andthermally joining the solid-phase ink with the receiving substrate orthe object, thereby forming a microassembly of heterogeneous materials,wherein the first material comprises SU-8 and the second material isselected from the group consisting of: SiO₂, Si, and Au.
 2. The methodof claim 1, further comprising repeating the contacting and stamping toadd additional solid-phase inks to the receiving substrate.
 3. Themethod of claim 1, wherein the stamp comprises a material withviscoelastic properties.
 4. The method of claim 1, wherein the objectcomprises a previously deposited solid-phase ink.
 5. The method of claim1, wherein the thermal joining comprises a method selected from thegroup consisting of: fusion bonding, eutectic bonding and adhesivebonding.
 6. The method of claim 1, wherein the thermal joining takesplace at a temperature in a range from about 150° C. to about 1000° C.7. The method of claim 1, wherein no external pressure is applied duringthe thermal joining.
 8. The method of claim 1, wherein an interfacebetween the solid-phase ink and the receiving substrate or thesolid-phase ink and the object exhibits a joining strength in a rangefrom about 0.3 J/m² to about 4.5 J/m².
 9. The method of claim 1, furthercomprising fabricating the solid-phase ink on the donor substrate.
 10. Amethod for microassembly of heterogeneous materials, the methodcomprising: fabricating a solid-phase ink comprising a polymer on adonor substrate, wherein the fabricating comprises: coating asacrificial layer onto a silicon substrate; coating a polymeric materialon the sacrificial layer and curing to form a polymer layer; patterningthe polymer layer to form a polymeric ink pattern; and removing thesacrificial layer, thereby forming the solid-phase ink comprising thepolymer for transfer printing; contacting a stamp with the solid-phaseink disposed on the donor substrate to form an inked stamp, where thesolid-phase ink is reversibly bound to the stamp; stamping the inkedstamp onto a receiving substrate or onto an object on the receivingsubstrate, wherein the receiving substrate or the object comprises amaterial different from the polymer; removing the stamp, therebytransferring the solid-phase ink to the receiving substrate; andthermally joining the solid-phase ink with the receiving substrate orthe object, thereby forming a microassembly of heterogeneous materials.11. The method of claim 10, wherein the sacrificial layer comprisespolymethylmethacrylate (PMMA), and wherein the polymeric materialcomprises an epoxy-based polymer.
 12. The method of claim 10, whereinthe polymeric material comprises SU-8, and wherein the patterningcomprises exposing the polymer layer to light through a pattern mask,baking after the exposing, and developing after the baking, therebyforming the polymeric ink pattern.
 13. The method of claim 10, whereinthe sacrificial layer is removed by exposure to a solvent.
 14. A methodfor microassembly of heterogeneous materials, the method comprising:fabricating a solid-phase ink comprising SiO₂ on a donor substratewherein the fabricating comprises: growing a SiO₂ layer on a siliconlayer; patterning the SiO₂ layer to form a SiO₂ ink pattern on thesilicon layer; patterning the silicon layer to form a silicon patternlaterally surrounding and underlying the SiO₂ ink pattern, therebyexposing a buried oxide layer away from the SiO₂ ink pattern; coveringthe SiO₂ ink pattern with photoresist and forming one or morephotoresist anchors to the buried oxide layer; and removing the siliconpattern, thereby forming the solid-phase ink comprising SiO₂ fortransfer printing; contacting a stamp with the solid-phase ink disposedon the donor substrate to form an inked stamp, where the solid-phase inkis reversibly bound to the stamp; stamping the inked stamp onto areceiving substrate or onto an object on the receiving substrate,wherein the receiving substrate or the object comprises a materialdifferent from the SiO₂; removing the stamp, thereby transferring thesolid-phase ink to the receiving substrate; and thermally joining thesolid-phase ink with the receiving substrate or the object, therebyforming a microassembly of heterogeneous materials.
 15. The method ofclaim 14, wherein the patterning of the silicon layer is carried outwith a mask pattern larger in lateral dimension than that employed toform the SiO₂ ink pattern.
 16. The method of claim 14, wherein thepatterning of the silicon layer comprises using reactive ion etching.17. The method of claim 14, wherein a silicon-on-insulator wafercomprises the silicon layer and the buried oxide layer.